Recently completed studies demonstrated that AII causes enhanced collagen- induced platelet aggregation in hypertensive patients. In a limited number of patients, both converting enzyme inhibitors (captopril) and beta-blockers (atenolol) blocked the AII-augmentation in platelet aggregation. This proposal will examine the effect of captopril compared to placebo, and compared to a positive (hydrochlorothiazide, which raises plasma AII levels) control on AII induced platelet aggregation in a larger patient population.